Shielded cable

Shielded cable on a Canadian fire alarm job is normally a twisted shielded pair used on data circuits, with a foil or braid layer and a bare drain wire wrapped around the conductors to intercept electrical interference. It is grounded at one end only, normally at the control unit, because a shield grounded at both ends becomes a parallel path between two points at different potential and produces intermittent communication faults and phantom ground faults. We could not locate any requirement in CAN/ULC-S524 or in Section 32 of the Canadian Electrical Code that says when a shield is required or how it is terminated: the free Canadian sources defer to the equipment manufacturer's installation instructions, and in practice those instructions are the operative document.

Last updated: September 2026

What it is and where it turns up

A shield is a conductive layer, usually an aluminium foil wrap with a bare drain wire lying against it, sitting between the jacket and the conductors. Its job is to intercept electromagnetic interference before it reaches the pair and give that interference a path back to one reference point.

On a Canadian fire alarm job you will almost always meet it on data circuits rather than on initiating or notification circuits. Twisted shielded pair is typically specified for RS-485 style buses between a control unit and its transponders or annunciators, with power run as a separate pair. That is the data communication link, and on some systems the addressable signalling line circuit.

Where the requirement comes from, and where the gap is

The honest part first. We could not locate a requirement in CAN/ULC-S524, or in Section 32 of the Canadian Electrical Code, that specifies when a shield is required or how it is to be terminated. S524 is copyrighted and paywalled, we have not read it, and we do not reproduce it. The free Canadian secondary sources we did read, including the Fire Protection Technicians Network's field device installation guideline, hand the question to the equipment manufacturer's installation instructions. Treat those instructions as the operative document and keep a copy on the job, because that is what you will be asked to produce.

What is not in doubt is everything that applies to any fire alarm conductor. Under Section 32 the conductors are copper, insulation rated not less than 300 V, and kept entirely independent of all other wiring, with no shared raceway, box or enclosure except at the defined connection points. The circuit is treated as Class 1 even at 24 V, because a fire alarm is a safety control device. A shield changes none of that.

The cable itself is built to CSA C22.2 No. 208, Fire alarm and signal cable, whose published scope covers single and multiple conductor cable at a maximum 300 V and a minimum 60 degrees C, in the types FAS, FAS 90, FAS 105, FAS 150 and FAS 200. Flame spread is a separate CSA FT number on the jacket, never an American CMP or CMR designation. Fire alarm cable in Canada works through the markings.

The field error that costs you a morning

Ground the shield at one end only, normally at the panel.

Ground it at both ends and the shield becomes a parallel path between two points that are not at the same potential. Current circulates through it, and the symptoms are the ones that eat a morning: intermittent communication faults on the data bus, a ground fault that appears and clears with no obvious cause, and behaviour that changes when unrelated equipment on the same building steel switches on.

The identical failure happens with nobody grounding anything on purpose. A drain wire left long inside a device box, touching the box or a bonded mounting screw, is one accidental second ground. The discipline is to cut the drain back and insulate it at every termination except the one you chose.

What else goes wrong

Continuity broken at a splice. A shield not carried through a junction box protects the run up to that box and nothing past it.

Data and power in one common jacket. Running the bus and its supply together is a reliable way to manufacture faults nobody can find.

A shield treated as a flame rating. Shielding says nothing about flame spread. Ordering shielded cable in the wrong FT grade for a return air plenum is a separate and expensive failure, discovered after the ceiling is closed.

Neighbouring terms

If you are chasing a fault that matches the description above, ground fault troubleshooting walks through isolating it circuit by circuit.

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